Gamma-glutamyl transferase is a valuable Marker in the study of the mechanism of experimental chemical carcinogenesis in rat liver and probably also in the colon. Using histochemical method we expect to be able to identify cellular precursors of azodye-induced liver carcinomas and 1,2-dimethylhydrazine-induced adenocarcinomas of rat colon. The change of inducible GT-ase into constitutive enzyme with the gradual increase of the half-life of GT-ase will be studied, with consideration of similar changes in certain selected groups of cellular consituents, especially messenger RNAs and the group of non-histone, acidic, nuclear proteins.